1. Field of the Invention
The invention relates to a system and method for modulating a combined light information signal and light carrier wave signal for transmission through an optical transmission link and demodulating the combined signal after reception to extract the transmitted information. More particularly, the invention relates to such a system and method of extremely simple and robust construction and which allow use of commercially available lasers and low frequency switching electronics for conversion of electronic domain information input signals to light signals and for retrieval of information from the light signals with reconversion to the electronic domain.
2. State of the Art
Optical communication systems allow information to be transmitted in the form of light. Fibre optic cables may be used to transmit the information from a transmitter to a receiver. Fibre optic cables can transmit light at extremely high speed and with relatively small power loss.
Referring to FIG. 1, a typical fibre optical cable consists of an optical core 14 surrounded by an optical cladding 16. The light is transmitted through core 14. As used herein, "light" refers to electromagnetic radiation that may be effectively transmitted through fibre optic cable and associated components, or other optic transmission systems known or contemplated in the art.
All materials that allow the transmission of electromagnetic radiation including light have an associated refractive index n, which is the ratio of the speed of light in a vacuum to the speed of light in the material. The speed of light in a vacuum is normalized to 1. The speed of light in a vacuum is constant regardless of the wavelength of the light. By contrast, the speed of light in a material is a function of wavelength and the structure of the material. Accordingly, the refractive index is a function of the wavelength of the light and the structure of the material.
Refraction refers to bending of light due to variations in the refractive index. As a ray of light passes from one material (or a vacuum) to another material, it is possible for the ray to refract, reflect, or partially refract and partially reflect. (The ray may also be partially absorbed.) Refracted rays are sometimes called transmitted rays, which term will be used herein to avoid confusion of subscripts.
The following three laws govern the relationship between incident, reflected, and transmitted (refracted) rays. First, the incident, reflected, and transmitted rays all reside in a plane, known as the plane of incidence, which is normal to the interface of the materials. Second, the angle of incidence .theta..sub.I equals the angle of reflection .theta..sub.R, where each angle is measured with respect to a line normal to the interface. Third, the angle of incidence .theta..sub.I and the angle of transmittance .theta..sub.T are related by Snell's law shown in equation (1), below: EQU n.sub.I sin .theta..sub.I =n.sub.T sin .theta..sub.T (1),
where n.sub.I is the refractive index of the material through which the incident ray travels, n.sub.T is the refractive index of the material through which the transmitted ray travels, .theta..sub.I is the angle of the incident ray with respect to the normal, and .theta..sub.T is the angle of the transmitted ray with respect to the normal.
An example of refraction is shown in FIGS. 2A and 2B. Referring to FIGS. 2A and 2B, a ray travels from Material A, having refractive index n.sub.I, to Material B, having a refractive index n.sub.T. The ratio of the angle of incidence .theta..sub.I to the angle of transmittance .theta..sub.T is governed by Snell's law, shown in equation (1). Generally, where n.sub.T &gt;n.sub.I (as in FIG. 2A), .theta..sub.T &lt;.theta..sub.I. Where n.sub.T &lt;n.sub.I (as in FIG. 2B), .theta..sub.T &gt;.theta..sub.I. (Of course, a larger .theta..sub.I also results in a larger .theta..sub.T.) At .theta..sub.T =90.degree., .theta..sub.I is defined to be at critical angle, denoted .theta..sub.C. The critical angle .theta..sub.C is defined in equation (2) below: EQU .theta..sub.C =sin.sup.-1 (n.sub.T /n.sub.I) (2),
For .theta..sub.I &gt;.theta..sub.C, all of the incident ray is totally internally reflected, remaining in the incident medium. An ideal fibre optic cable has total internal reflection, which leads to a relatively small amount of loss in the transmission of light through the cable.
Referring to FIG. 3, one end of fibre optic cable 10 interfaces with air, which has a refractive index n.sub.1 (which happens to be about 1.00027). Core 14 has a refractive index n.sub.2, where n.sub.2 &gt;n.sub.1. Cladding 16 has a refractive index n.sub.3. Dashed lines show the normal with respect to the air-core interface and the core-cladding interface. An incident ray hits the air-core interface at angle .theta..sub.I1. The transmitted (refracted) ray is referred to as ray TI to designate the ray as both a transmitted ray with respect to the air-core interface and an incident ray with respect to the core-cladding interface. The angle of transmittance .theta..sub.T may be derived according to Snell's law, shown in equation (1).
An angle of incidence .theta..sub.I2 inside core 14 equals 90.degree. minus .theta..sub.T. If .theta..sub.I &gt;.theta..sub.C, there will be total internal reflection and ray TI will continue to transmit through core 14 at angle .theta..sub.I2 until another interface is reached. Further, there is no loss of radiated power at the reflection (although there is loss as the light passes through core 14).
If .theta..sub.I is too large, .theta..sub.I2 cannot be greater than .theta..sub.C, and there will not be total internal reflection. The maximum incident angle .theta..sub.MAX is derived in equation (3), below: EQU .theta..sub.MAX =sin.sup.-1 ((1/n.sub.1)(n.sub.2.sup.2 -n.sub.3.sup.2).sup.1/2) (3),
where n.sub.1, n.sub.2, and n.sub.3 are the refractive indices defined above in connection with FIG. 3. Accordingly, if .theta..sub.I &gt;.theta..sub.MAX, there will not be total internal reflection.
Interference refers to the consequence which arises when two light waves starting from the same point source or from two identical point sources arrive at some point P after having travelled two trajectories with different lengths. Generally, the two light waves have the same frequency, but different phases at the time they reach point P. However, the inventor has discovered that it is possible to employ the interference phenomenon with laser light waves of different frequencies and from different sources, as the description of the present invention will hereinafter show.
Modulation is used to impress information from one signal into another signal to create a modulated signal. There are various types of modulation, including amplitude modulation and frequency modulation.
Amplitude modulation is a method of transmitting an information signal by superimposing it on a carrier signal which has a much higher frequency. Consider the following simple example. A carrier signal cos .omega..sub.C t is varied in amplitude by a modulating information signal cos .omega..sub.M t, where .omega..sub.M is much less than .omega..sub.C. The resulting modulated signal I.sub.Mod is shown in equation (4), below: EQU I.sub.Mod =(1+M cos .omega..sub.M t)cos .cndot..sub.C t (4),
where M is the modulating index, which is less than or equal to 1, .omega..sub.M =2.pi.f.sub.M =2.pi./.lambda..sub.M, and .omega..sub.C =2.pi.f.sub.C =2.pi./.lambda..sub.C. I.sub.Mod may be rewritten as in equation (5), below: EQU I.sub.Mod =cos .omega..sub.C t+m/2(cos(.omega..sub.C +.omega..sub.M)t+cos(.omega..sub.C -.omega..sub.M)t) (5).
Equation (5) illustrates that the modulated carrier has power at frequencies .omega..sub.C, .omega..sub.C +.omega..sub.M, and .omega..sub.C -.omega..sub.M. In amplitude modulation, the frequency of the information signal remains constant while the amplitude varies to convey information. In frequency modulation, the frequency of the modulated signal varies depending on the frequency of the information signal.
Where the information (modulating) signal is a complex waveform f(t), the amplitude modulated waveform may be (K+f(t))*cos .omega..sub.C t, where K is a constant that is large enough such that (K+f(t)) is never negative.
In many circumstances, the modulated signal I.sub.Mod can be transmitted more easily and efficiently than can the information signal cos .omega..sub.M t. At the conclusion of the transmission, a receiver strips the carrier wave, leaving only the information wave.
Systems are known in the art to modulate and demodulate light signals for information transmission purposes. However, such state-of-the-art systems are complex, expensive and require relatively sophisticated electronic processing to provide a modulated light output signal and to retrieve an electronic signal at the receiving end of the transmission.